Chronic hepatitis C virus (HCV) infection is a serious worldwide health problem. HCV is a positive-strand RNA virus member of the Flaviviridae family (Choo et al., 1989, Science 244:359-362), and is one of the leading causes of chronic liver disease (Tong et al., 1995, N. Engl. J. Med. 332:1463-1466). Additionally, chronic HCV infection has been associated with autoimmune syndromes, immune complex disorders, and mixed cryoglobulinemia (McMurray et al., 1998, Rheum. Dis. Clin. North Am., 24:353-374; Zignego et al., 1999, J. Hepatol., 31:369-376).
One of the most striking features of HCV is its ability, in most instances, to circumvent eradication by the immune system. It is estimated that up to 75% of patients infected with HCV become chronically infected (Tong et al., supra; Alter et al., 1992, N. Engl. J. Med., 327:1899-1905; Heintges et al., 1997, Hepatology, 26:521-526; Seeff, 1995, Semin. Gastrointest. Dis., 6:20-27) despite the fact that most patients generate HCV-specific antibodies (Abe et al., 1992, Hepatology, 15:690-695; Bradley et al., 1990, Gastroenterology, 99:1054-1060; Farci et al., 1992, J. Infect. Dis., 165:1006-1011; Hilfenhaus et al., 1992, J. Gen. Virol., 73:1015-1019; Shimizu et al., 1990, Proc. Natl. Acad. Sci. USA, 87:6441-6444), as well as CD4+ and CD8+ T cell responses (Koziel, 1997, J. Viral Hepat., 4 Suppl. 2:31-41; Koziel et al., 1993, J. Virol., 67:7522-7532; Schupper et al., 1993, Hepatology,18:1055-1060; Ferrari et al., 1993, Hepatology, 19:286-295). There is evidence, however, that humoral and T cell-mediated immune responses to HCV infection can, at least in some instances, determine the outcome of HCV infection and disease (Zibert et al., 1997, Hepatology, 25:1245-1249; Farci et al., 1996, Proc. Natl. Acad. Sci. USA, 93:15394-15399; Cooper et al., 1999, Immunity, 10:439-449; Missale et al., 1996, J. Clin. Invest., 98:706-714).
In spite of recent progress in the management of chronic HCV disease, the current therapies for chronic HCV infection often do not result in viral clearance (Main et al., 1998, Antivir. Chem. Chemother., 9:449-460; Gish, 1999, Semin. Liver Dis., 19 Suppl. 1:35-47). Several theories have been proposed to explain this lack of clearance including the development of immunological tolerance to HCV antigens (for a review, see Cerny & Chisari, 1999, Hepatology, 30:595-601).
The induction of immunological tolerance in animals is known. The liver is recognized to play an important role in immunological tolerance induction. Cantor & Dumont (1967, Nature 215:744-745) showed that the liver was important to the tolerogenic effect of oral feeding. Introduction of antigens into the portal vein (PV), which leads to the liver, has been shown to induce tolerance. For example, tolerance is induced following PV injection of sheep red blood cells (Triger et al., 1973, Immunology, 25:941-950), schistosome eggs (Cuison et al.,1995, Int. J. Parasitol., 25:993-998), whole allogeneic cells (Gorczynski, 1995, Cell. Immunol., 160:224-231; Sugiura et al., 1997, Immunobiology, 197:460-477), and allogeneic class I heavy chain proteins (Wang et al., 1996, Transplantation, 61:448-457. Shimizu et al. (1998, J. Immunol., 161:4520-4521) studied germline-transmissible, transgenic mouse models of hepatitis B virus (HBV) infection that are tolerant to HBV surface antigen (HBsAg), expressed in the liver from birth.
Although most organ grafts between MHC mismatched individuals are rapidly rejected unless the recipient is immunosuppressed, liver-allografts in various animal species can induce tolerance to themselves and to subsequent allogeneic grafts (Calne et al., 1969, Nature, 223:472-476; Qian et al., 1994, Hepatology, 19:916-924; Sriwatanawongsa et al., 1995, Nat. Med., 1:428-432). In these animal models, tolerance induction occurs after an initial host response against the graft, followed by acceptance (Millard et al., 1971, Transplant. Proc., 3:505-508; Kamada et al., 1983, Transplantation, 35:304-311). There is also clinical evidence that a similar state of unresponsiveness/tolerance by liver grafts is gradually induced in some human recipients (Starzl, 1998, Transplant. Proc., 30:3845). Furthermore, there is an indication from studies in mice that perpetuation of T cell tolerance is dependent on persistence of the tolerizing antigen (Ehl et al., 1998, Nat. Med., 4:1015-1019).
Models for tolerance to HCV have not been developed. A number of HCV germline-transmissible transgenic mouse lines have been developed, expressing different HCV antigens in the liver and other tissues. Koike et al. (1995, J. Gen. Virol., 76:3031-8) developed mice transgenic for the HCV envelope proteins, E1 and E2, under the control of the hepatitis B virus (HBV) regulatory region. Moriya et al. (1997, J. Gen. Virol., 78:1527-31) generated mice transgenic for the HCV core protein, also under the control of the HBV regulatory region. Pasquinelli et al. (1997, Hepatology, 25:719-27) generated transgenic mice expressing the HCV core protein and a carboxy-terminally-truncated E2 protein in the liver, under the control of the liver specific mouse urinary protein and albumin promoters, respectively. Kawamura et al. (1997, Hepatology, 25:1014-21) generated mice transgenic for a cassette of core, E1, and E2 genes, under the control of either the mouse major urinary promoter or the albumin promoter. Additionally, Wakita et al. (1998, J. Biol. Chem., 273:9001-6) developed a germline transgenic mouse, using the cre/lox system, for the inducible expression of HCV proteins (C, E1, E2 and NS2) in the adult animal, to study the immune response to and pathogenesis of HCV infection. All non-inducible germline transgenic HCV models, however, would be expected to be inherently “tolerant” to the particular HCV antigen expressed, as the mice express the proteins at birth and their immune systems see them as “self.”
A model of tolerance to HCV that mimics the course of tolerance that develops in the natural progression to chronic HCV status is needed. The present invention is directed to this model of tolerance and other needs.